Metal nanoparticles are utilized as not only alloy particles for sintering to produce a thermoelectric conversion material, but also as a three-way catalyst, photocatalyst, or other catalyst and other functional powder.
As the conventional method for producing metal nanoparticles which contain several types of metal, for example, there is the method of adding a reducing agent into a solution which contains several types of metal compounds so as to cause metal nanoparticles to precipitate.
For example, in the conventional method for producing Bi and Te composite metal nanoparticles for a thermoelectric conversion material, NaBH4 or another reducing agent is added to a solution of BiCl3, TeCl4, or another metal compound to cause Bi and Te composite metal nanoparticles to precipitate.
In recent years, there has been known the so-called “solution plasma method” which generates plasma in a solution which contains a metal compound and utilizes the reducing action of plasma to cause metal nanoparticles to precipitate.
For example, PLT 1 describes a method of causing generation of plasma in a solution which contains a metal oxoacid to produce metal oxide nanoparticles. Further, PLT 2 describes a method of causing generation of plasma in an aqueous solution of a metal salt to produce metal nanoparticles of a particle size of 500 nm or less.
Further, there is known a method of producing metal nanoparticles which utilizes a flow type reaction apparatus, for example, a microreactor.
For example, PLT 3 describes a method of producing metal nanoparticles by mixing a hydrazine solution in an aqueous solution which contains a metal salt to form a hydrazine complex in a microreactor and reducing the obtained hydrazine complex by an alkali solution. Further, PLT 4 describes a method of producing metal nanoparticles by firing one or more energy beams among laser beams, electromagnetic waves, particle beams, or ultrasonic waves at a starting solution which is supplied in a microreactor.
As one form of the metal nanoparticles, core-shell type metal nanoparticles are known.
For example, PLT 5 describes utilizing the so-called “hot soap method”, that is, injecting ZnO nanoparticles for forming the cores and the CoSb3 precursor for forming the shells into a dispersant which is heated to a high temperature and covering the ZnO by CoSb3, so as to produce core-shell type metal nanoparticles.